Distribution of spin dipole transition strength in the 15N(n,p)15C reaction.

Abstract

The reaction $^{15}\mathrm{N}$(n,p${)}^{15}$C was studied at a neutron energy of 288 MeV using the TRIUMF (n,p) charge-exchange facility and a high-pressure gas target. The angular distributions for spin dipole (\ensuremath{\Delta}L=1) transitions to the states in $^{15}\mathrm{C}$ at energies 0 and 0.740 MeV, as well as for higher excitation energies, were measured and the results were compared with distorted-wave impulse approximation calculations. The measured distribution of the spin dipole strength agrees well with shell-model predictions, indicating that a rather simple model provides a satisfactory description of the $^{15}\mathrm{N}$ ground state, and of positive-parity states in $^{15}\mathrm{C}$ up to about 18 MeV excitation. The magnitude of the peak cross sections (at \ensuremath{\simeq}7\ifmmode^\circ\else\textdegree\fi{}) is described well by the calculations when the theoretical cross section is renormalized by a factor of 0.7. The calculated cross sections near 0\ifmmode^\circ\else\textdegree\fi{} are generally smaller than experimental data. If this is a general feature of \ensuremath{\Delta}L=1 transitions, it suggests that estimates of Gamow-Teller strength based on a multipole decomposition of measured cross sections may be too high.